In offset lithographic printing, each printing stage includes a plate cylinder, to which the printing plates are fastened tightly around the circumference. The plate cylinder is equipped with superimposed inking, watering and wiping mechanisms. The plate cylinder does not come into contact with the sheets of material to be printed. Instead, the plate cylinder transfers the image to an intermediate blanket cylinder which has a specially composed smooth rubber blanket surface. During operation, the blanket cylinder is packed with a sheet having a cross-sectional area slightly longer and wider than the sheets to be printed which is disposed between an underlying steel cylinder and the rubber surface coating to lift the rubber surface which is to receive the impression from the printing plate slightly radially outward from portions of the rubber surface which are not packed. The blanket cylinder receives the impression from the plate cylinder and transfers it or offsets it onto the sheets to be printed while being carried around the blanket cylinder located out of contact with the plate cylinder.
In many applications it is desirable to apply a coating to a printed sheet. For example, a water soluble polymer finish may be applied to a work piece printed by offset lithography to "dry" the sheet quickly by coating the surface while it is still tacky. This coating method avoids the need for powder driers that may be cumbersome or air drying procedures which may be slow. Furthermore, coatings are also useful for providing a glossy finish that improves the rub resistance of the coated sheet and improves its overall appearance. Finally, adhesive coatings may be applied to printed packaging. Heat set adhesives may be applied to enable attachment of a feature such as the clear plastic bubble of a package used to display the product.
Application of coatings to a work piece is made difficult by various requirements. For example, the coating should be uniform and its thickness should be controlled. Moreover, the coatings should be applied quickly, before its vehicle evaporates causing it to thicken. In many applications it is desirable to apply an overall coating to the printed sheet. For example, a UV curable or water-soluble polymer finish may be applied to a workpiece printed by offset lithography. The coating on the sheet is quickly dried while the surface of the ink is still tacky.
Printing presses conventionally have a liquid trough and train of smooth surface transfer rollers located above the blanket cylinder and the plate cylinder for supplying of other liquids such as varnishes to the printed sheet via the blanket cylinder.
Furthermore, add-on coating units have been developed which are attached to the last printing station of a lithographic press for selective positioning with the blanket cylinder which during normal printing transfers the image of the material to be printed under pressure at a nip produced by an impression cylinder. A supply of coating material is transferred from a metering station which may be an engraved roller contacting the trough of material and a doctor blade which produces a uniform coating on the engraved roller as it rotates past the doctor blade for contact with the blanket cylinder. The add-on units operate under the principle of selective positioning of a coating material metering device in contact with the blanket cylinder to transfer the coating material to the blanket cylinder to permit coating when sheets are run through the nip between the blanket cylinder and impression cylinder or withdrawal so that the last printing station may be used in its conventional manner for printing.
U.S. Pat. Nos. 4,270,483, 4,685,414, 4,825,804, 4,934,305 and 5,107,790 describe systems for applying coating materials used in conjunction with lithographic printers. U.S. Pat. Nos. 4,685,414 and 4,825,804 are incorporated by reference herein in their entirety. Each of the coating systems described in the aforementioned patents is operated in conjunction with a lithographic printing press of standard design. These systems, while successfully performing coating of printed sheets either in their entirety or in spot applications, suffer from the disadvantage of being of substantial cost because of the requirement to be added on to an offset printing system which itself is a very expensive device. Costs for add-on coating systems may exceed $100,000.
U.S. Pat. No. 4,270,483 discloses the transfer of coating material from a pickup roller to an application roller which in turn transfers the coating material to a blanket cylinder. Coating is performed when the top face of the sheet to be coated passes between the impression roller and the blanket cylinder which are opposed to each other to form a nip.
U.S. Pat. No. 4,685,414 discloses the transfer of coating material from the engraved surface of a roller which meters coating material by providing a continuous excessive flow of coating material between a doctor blade and the engraved surface of the roller which is metered by the engraved surface and doctor blade to produce a uniform coating on the surface of the engraved roller which contacts a blanket roller. The blanket roller in turn contacts an impression roller to form a nip between which the sheets to be coated. Rolling contact of the blanket roller on the first face of the sheets to be coated transfers a uniform thickness of coating material to the sheets as they pass the nip between the blanket cylinder and the impression roller.
U.S. Pat. No. 4,825,804 discloses an add-on coating system in accordance with the '414 patent which facilitates the ability of the final press unit to perform both printing and coating functions. The mounting assembly enables the operator to disengage the coating assembly and move it away from the final press unit both horizontally and vertically so that the unit may be used as a conventional lithographic press unit.
U.S. Pat. No. 4,934,305 discloses an add-on coating system which utilizes a transfer roller to which is fed coating material which is metered to produce a uniform thickness for transfer to a blanket cylinder. Rolling contact between the blanket cylinder and an impression roller transfers coating material coated on the top surface of the blanket cylinder to a first face of sheets to be coated as they pass between a nip between the blanket cylinder and the impression roller.
U.S. Pat. No. 5,107,790 discloses a coating system which permits coating rollers to be moved into contact with the plate cylinder and blanket cylinder of a conventional printing unit and to be withdrawn to accessible positions for cleaning when not in use. Coating material is applied from a coating roller to a pickup roller which is coated with a material such as lacquer. The pickup roller is positioned in a withdrawn position from contact with the plate cylinder to permit spot cleaning and positioned in surface contact with the plate cylinder during operation. The coating rollers have a larger diameter which reduces the tendency of the rollers to sling coating material off the surface by centrifugal force. The coating material is in turn transferred from the blanket cylinder to a first surface of a sheet to be coated which passes between a nip formed between the blanket cylinder and the impression roller.
An in-line coater is sold by Norton Burdette Company of Nashua, N.H., which has a single roller driven directly by a D.C. motor. The roller is a gravure cylinder that transfers coating to a standard press unit blanket cylinder. The coater is attached to a pivoting arm and the unit is pivoted away from the press unit when the coater is not in use.
An in-line coater is sold by IVT Color Dry Ink of Fairfield, Conn., which applies coating from a reservoir pan to a standard press unit blanket cylinder using a pickup roller that delivers the coating material supplied to a metering device roller. The metering device roller applies the coating to the blanket cylinder of a press unit.
Each of the aforementioned coating systems utilizes the blanket cylinder to transfer coating material to the face of the sheets to be coated with the coating material being applied to the blanket cylinder surface from a metering device. The coating material is transferred from the raised surface of the blanket cylinder, produced by the aforementioned packing the blanket cylinder, to a first face of the sheet to be coated as the sheets pass between a nip of the packed blanket cylinder and the impression roller. As a result of the packing of the blanket cylinder with the underlying sheet, contact with the metering device only coats the raised surface of the blanket roller. Systems which apply coating material to a first face of sheets from a coated portion of the cross-sectional area of a blanket cylinder which is raised with respect to the remainder of the cross-sectional area of the blanket cylinder do not have a problem of transferring coating material to the second face of the sheets as a result of contact of the blanket cylinder with the impression rollers at gaps at least between the sheets. However, as has been stated above with respect to the cost of add-on systems, they are extremely expensive because of their design requiring accommodation with the existing expensive parts of the last printing station of a lithographic offset printing system.